The number of echoes combined into a single image or image set in multiple echo imaging sequences or echo-train techniques such as rapid acquisition with relaxation enhancement (RARE), fast-spin echo (FSE), and turbo spin-echo (TSE). In RARE imaging, the ETL typically equals the acceleration factor.
As for tooth pain, it is caused by a reaction of the nerves inside a tooth's pulp chamber with the severity dependent upon the type and degree of the stimulus. What follows are some examples of symptoms you may be feeling and their possible causes.
Toothache occurs from inflammation of the central portion of the tooth called pulp. The pulp contains nerve endings that are very sensitive to pain. Inflammation to the pulp or pulpitis may be caused by dental cavities, trauma, and infection. Referred pain from the jaw may cause you to have symptoms of a toothache.